6.4.3 control of blood water potential

what is associated with nephrons

blood vessels

function of bowmans capsule

formation of glomerular filtrate (ultrafiltration)

function of proximal convoluted tube

reabsorption of water and glucose

function of loop of Henle

maintenance of a gradient of sodium ions in the medulla

function of distal convoluted tube

reabsorption of water

describe formation of glomerular filtrate

1. high hydrostatic pressure in glomerulus

   • as diameter of afferent arteriole is wider than efferent arteriole

2. small substances forced into glomerular filtrate, filtered by:

• pores between capillary endothelial cells

• capillary basement membrane

• podocytes

3. large proteins / blood cells remain in blood

describe reabsorption of glucose by proximal convoluted tube

1. Na+ actively transported out of epithelial cells to capillary

2. Na+ moves by facilitated diffusion into epithelial cells down a concentration gradient, bringing glucose against its concentration gradient

3. glucose moves into capillary by facilitated diffusion down its concentration gradient

describe reabsorption of water by proximal convoluted tube

glucose lowers water potential

water moves via osmosis down a water potential gradient

describe and explain features of cells in the PCT that allow rapid reabsorption of glucose in the blood

microvilli - provides a large SA

many channel/carrier proteins - for facilitated diffusion / co-transport

many carrier proteins - for active transport

many mitochondria - produce ATP for active transport

many ribosomes - produces channel/carrier proteins

suggest why glucose in found in urine of untreated diabetic person

blood glucose concentration is too high so not all glucose is reabsorbed at PTC

as glucose carrier/ cotransporter proteins are saturated

explain the importance of maintaining a gradient of sodium ions in the medulla

water potential decreases down the medulla

water potential gradient is maintained between the collecting duct and medulla

to maximise reabsorption of water by osmosis from filtrate

role of loop of Henle in maintaining a gradient of sodium ions in the medulla

in the ascending limb:

• Na+ is actively transported out

• water remains as ascending limb is impermeable to water

• this increases concentration of Na+ in the medulla, lowering water potential

in the descending limb:

• water moves out by osmosis then reabsorbed by capillaries

• Na+ recycled - diffuses back in

why do animals that need to conserve water have a long loop of Henle

more Na+ moved out - Na+ gradient is maintained for longer in medulla

so water potential gradient is maintained for longer

so more water can be reabsorbed from collecting duct by osmosis

describe reabsorption of water by the distal convoluted tube and collecting ducts

water moves out of distal convoluted tube and collecting duct via osmosis down a water potential gradient

controlled by ADH which increases their permeability

what is osmoregulation

control of water potential of the blood

describe the role of the hypothalamus in osmoregulation

contain osmoreceptors which detects increase or decrease in blood water potential

produces more ADH when water potential is low or less ADH when water potential is high

describe the role of the posterior pituitary gland in osmoregulation

secretes ADH into blood due to signals from hypothalamus

describe the role of ADH in osmoregulation

attaches to receptors on colleting duct

stimulating addition of channel proteins into CSM

so increases permeability of cells of collecting duct and DCT to water

so increases water reabsorption from collecting duct by osmosis

so decreases volume and increases concentration of urine produced